从 Java 到 Kotlin - 从 Java 到 Kotlin 的速查表.zip

<p align="center"> <img alt="FromJavaToKotlin" src="https://raw.githubusercontent.com/amitshekhariitbhu/from-java-to-kotlin/master/assets/from_java_to_kotlin.png"> </p> # From Java To Kotlin > From Java To Kotlin - Your Cheat Sheet For Java To Kotlin ## About me Hi, I am Amit Shekhar, Co-Founder @ [Outcome School](https://outcomeschool.com) • IIT 2010-14 • I have taught and mentored many developers, and their efforts landed them high-paying tech jobs, helped many tech companies in solving their unique problems, and created many open-source libraries being used by top companies. I am passionate about sharing knowledge through open-source, blogs, and videos. You can connect with me on: - [Twitter](https://twitter.com/amitiitbhu) - [YouTube](https://www.youtube.com/@amitshekhar) - [LinkedIn](https://www.linkedin.com/in/amit-shekhar-iitbhu) - [GitHub](https://github.com/amitshekhariitbhu) ## Join **Outcome School** and get high paying tech job: [Outcome School](https://outcomeschool.com) ## Print to Console > Java ```java System.out.print("Amit Shekhar"); System.out.println("Amit Shekhar"); ``` > Kotlin ```kotlin print("Amit Shekhar") println("Amit Shekhar") ``` --- ## Constants and Variables > Java ```java String name = "Amit Shekhar"; final String name = "Amit Shekhar"; ``` > Kotlin ```kotlin var name = "Amit Shekhar" val name = "Amit Shekhar" ``` --- ## Assigning the null value > Java ```java String otherName; otherName = null; ``` > Kotlin ```kotlin var otherName : String? otherName = null ``` --- ## Verify if value is null > Java ```java if (text != null) { int length = text.length(); } ``` > Kotlin ```kotlin text?.let { val length = text.length } // or simply val length = text?.length ``` --- ## Verify if value is NotNull OR NotEmpty > Java ```java String sampleString = "Shekhar"; if (!sampleString.isEmpty()) { myTextView.setText(sampleString); } if(sampleString!=null && !sampleString.isEmpty()){ myTextView.setText(sampleString); } ``` > Kotlin ```kotlin var sampleString ="Shekhar" if(sampleString.isNotEmpty()){ //the feature of kotlin extension function myTextView.text=sampleString } if(!sampleString.isNullOrEmpty()){ myTextView.text=sampleString } ``` --- ## Concatenation of strings > Java ```java String firstName = "Amit"; String lastName = "Shekhar"; String message = "My name is: " + firstName + " " + lastName; ``` > Kotlin ```kotlin var firstName = "Amit" var lastName = "Shekhar" var message = "My name is: $firstName $lastName" ``` --- ## New line in string > Java ```java String text = "First Line\n" + "Second Line\n" + "Third Line"; ``` > Kotlin ```kotlin val text = """ |First Line |Second Line |Third Line """.trimMargin() ``` --- ## Substring > Java ```java String str = "Java to Kotlin Guide"; String substr = ""; //print java substr = str.substring(0, 4); System.out.println("substring = " + substr); //print kotlin substr = str.substring(8, 14); System.out.println("substring = " + substr); ``` > Kotlin ```kotlin var str = "Java to Kotlin Guide" var substr = "" //print java substr = str.substring(0..3) // println("substring $substr") //print kotlin substr = str.substring(8..13) println("substring $substr") ``` --- ## Ternary Operations > Java ```java String text = x > 5 ? "x > 5" : "x <= 5"; String message = null; log(message != null ? message : ""); ``` > Kotlin ```kotlin val text = if (x > 5) "x > 5" else "x <= 5" val message: String? = null log(message ?: "") ``` --- ## Bitwise Operators > Java ```java final int andResult = a & b; final int orResult = a | b; final int xorResult = a ^ b; final int rightShift = a >> 2; final int leftShift = a << 2; final int unsignedRightShift = a >>> 2; ``` > Kotlin ```kotlin val andResult = a and b val orResult = a or b val xorResult = a xor b val rightShift = a shr 2 val leftShift = a shl 2 val unsignedRightShift = a ushr 2 ``` --- ## Check the type and casting > Java ```java if (object instanceof Car) { Car car = (Car) object; } ``` > Kotlin ```kotlin if (object is Car) { var car = object as Car } // if object is null var car = object as? Car // var car = object as Car? ``` --- ## Check the type and casting (implicit) > Java ```java if (object instanceof Car) { Car car = (Car) object; } ``` > Kotlin ```kotlin if (object is Car) { var car = object // smart casting } // if object is null if (object is Car?) { var car = object // smart casting, car will be null } ``` --- ## Multiple conditions > Java ```java if (score >= 0 && score <= 300) { } ``` > Kotlin ```kotlin if (score in 0..300) { } ``` --- ## Multiple Conditions (Switch case) > Java ```java int score = // some score; String grade; switch (score) { case 10: case 9: grade = "Excellent"; break; case 8: case 7: case 6: grade = "Good"; break; case 5: case 4: grade = "OK"; break; case 3: case 2: case 1: grade = "Fail"; break; default: grade = "Fail"; } ``` > Kotlin ```kotlin var score = // some score var grade = when (score) { 9, 10 -> "Excellent" in 6..8 -> "Good" 4, 5 -> "OK" else -> "Fail" } ``` --- ## For-loops > Java ```java for (int i = 1; i <= 10 ; i++) { } for (int i = 1; i < 10 ; i++) { } for (int i = 10; i >= 0 ; i--) { } for (int i = 1; i <= 10 ; i+=2) { } for (int i = 10; i >= 0 ; i-=2) { } for (String item : collection) { } for (Map.Entry<String, String> entry: map.entrySet()) { } ``` > Kotlin ```kotlin for (i in 1..10) { } for (i in 1 until 10) { } for (i in 10 downTo 0) { } for (i in 1..10 step 2) { } for (i in 10 downTo 0 step 2) { } for (item in collection) { } for ((key, value) in map) { } ``` --- ## Collections > Java ```java final List<Integer> listOfNumber = Arrays.asList(1, 2, 3, 4); final Map<Integer, String> keyValue = new HashMap<Integer, String>(); map.put(1, "Amit"); map.put(2, "Anand"); map.put(3, "Messi"); // Java 9 final List<Integer> listOfNumber = List.of(1, 2, 3, 4); final Map<Integer, String> keyValue = Map.of(1, "Amit", 2, "Anand", 3, "Messi"); ``` > Kotlin ```kotlin val listOfNumber = listOf(1, 2, 3, 4) val keyValue = mapOf(1 to "Amit", 2 to "Anand", 3 to "Messi") ``` --- ## for each > Java ```java // Java 7 and below for (Car car : cars) { System.out.println(car.speed); } // Java 8+ cars.forEach(car -> System.out.println(car.speed)); // Java 7 and below for (Car car : cars) { if (car.speed > 100) { System.out.println(car.speed); } } // Java 8+ cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed)); cars.parallelStream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed)); ``` > Kotlin ```kotlin cars.forEach { println(it.speed) } cars.filter { it.speed > 100 } .forEach { println(it.speed)} // kotlin 1.1+ cars.stream().filter { it.speed > 100 }.forEach { println(it.speed)} cars.parallelStream().filter { it.speed > 100 }.forEach { println(it.speed)} ``` --- ## Splitting arrays > java ```java String[] splits = "param=car".split("="); String param = splits[0]; String value = splits[1]; ``` > kotlin ```kotlin val (param, value) = "param=car".split("=") ``` --- ## Defining methods > Java ```java void doSomething() { // logic here } ``` > Kotlin ```kotlin fun doSomething() { // logic here } ``` ### Default values for method parameters > Java ```java double calculateCost(int quantity, double pricePerItem) { return pricePerItem * quantity; } double calculateCost(int quantity) { // default price is 20.5 return 20.5 * quantity; } ``` > Kotlin ```kotlin fun calculateCost(quantity: Int, pricePerItem: Double = 20.5) = quantity * pricePerItem calculateCost(10, 25.0) // 250 calculateCost(10) // 205 ``` --- ##